On the Audibility of Phase

Just found this compilation of references to the advantages of linear phase speakers (from various authors), on the Linkwitz Labs web site:

Click to access Attributes_Of_Linear_Phase_Loudspeakers.pdf

If you read it, you too might just be a little bit curious about how this stuff sounds..? What it is saying is that a conventional speaker no matter how high end, cannot rival a full-fat DSP-based speaker in terms of the ear’s two stage process of responding first to transients, and only then to the spectral information.

…there is a volume of research results that clearly indicates, that rather than asking “is phase distortion audible?” we should now be asking question “how does the phase distortion manifest itself?”….

“…..Another area in which loudspeakers are disreputable is in the neglect of the time domain. The traditional view is that all that matters is to be able to reproduce continuous sine waves over the range of human hearing.

A very small amount of research and thought will reveal that this is a misguided view. Frequency response is important, but not so important that the attainment of an ideal response should be to the detriment of realism. One tires of hearing that “phase doesn’t matter” in audio or “the ear is phase deaf”. These are outmoded views which were reached long ago in flawed experiments and which are at variance with the results of recent psychoacoustic research….

…Linear-phase loudspeakers offer everything that minimum-phase loudspeakers can offer, and then reward you with often vastly superior performance in time domain, as explained in the pages above.

It appears, that my poor and outdated listening/evaluating habits, coupled with lack of standard listening methodology for time/space-domain assessment of loudspeakers conspired to cloud my ability to really critically listen to the full set of my loudspeakers during some of my evaluation tests. …

…I pointed out earlier the effect of feeling closer to the orchestra, as if I could better discriminate their sitting arrangement. Both of these effects have really nothing to do with frequency domain – they are both more of the time/space domain phenomena.

…At the time of this writing, linear-phase loudspeakers are still a new “kid on the block”. Past attempts in creating them resulted in offerings that were simply too expensive for wide-spread use. The most accurate implementation of linear-phase loudspeaker requires a full set of individual driver measurements, coupled with a DSP approach, in addition to an active amplification system. This really makes the linear-phase system highly customized device – a world of difference in comparison to the current approach of loudspeaker industry….

It is clear, that designing loudspeakers using frequency-domain characteristics as the main (or only) criteria leads to stagnated, oversimplified, and ultimately inaccurate system. If I continued to design loudspeakers that never reveal time-domain or spatial-domain subtleties, I would never even know of the existence of such subtleties, therefore, I would never be motivated to change – thus allowing the vicious cycle to continue. It is evident, that the ear examines the incoming audio stimulus in two-stage process: (1) location – here the transient of the stimulus is examined, and (2) signal – here the spectral properties of the stimulus are examined. The two processes always work in-tandem. It is therefore essential, that the loudspeaker provides undistorted waveforms to the auditory system to enable correct processing of both stages.

So, here I am. Struggling to come out of the “frequency-domain box” and into the new world of time/frequency/space-domain characteristics of contemporary loudspeakers. But even at these early stages of adopting a new technology, I find it already very rewarding. This is because it’s evident that a new, accurate and realistic acoustic transduction technology is being achieved in much more accessible commercial way.

There’s lots more interesting stuff in the paper.

Vaughan Williams’ A Sea Symphony

Just playing this recording and I felt compelled to write something about how it has truly captured the… I was going to say “essence” of the performance, but I think that would be selling it short. In fact it seems to have actually captured the whole thing. Listening to it at realistic volume, it is as though I am there, and it is wonderful. The recording immerses me in a large, quiet space, populated with cultured people performing a spectacular piece of music that, nevertheless, is imbued with restraint and understatement. High culture, not just its essence, has been brought into my decidedly un-cultured surroundings.

If it could be quantified, it might be claimed that a recording could only capture a small percentage of what actually happened in that church in 1989. Listening on a vinyl gramophone or an ‘iDock’, I might agree, and with nothing else to compare it to, that would be that. But this was a digital recording, skilfully made so that the event was captured perfectly, and not a single atom of it has changed in the intervening quarter of a century. With the right playback equipment the performance can be re-lived, forever.

The Importance of Bass

It is only recently that I have come to realise how crucial bass is to the overall sound of an audio system. Over the years, I have had a succession of moderately-sized bass reflex speakers which never quite delivered the performance I might have hoped for. The bass reflex, or ported, configuration is extremely common, even in speakers that cost the price of a car, yet if you read about its pitfalls, it does seem amazing that anyone would consider it for serious use.

The big old speakers of our youth in the 1970s (if you’re as old as me!) were different: voluminous sealed enclosures with large woofers. The characteristics of these speakers are quite different from the modern ported bass reflex variety. Some of these differences are:

• For the same -3dB point, sealed enclosure speakers have to be considerably larger.
• Below the -3dB point, sealed speakers roll-off at 12dB/octave while bass reflex (ported) speakers roll off at 24dB/octave. A typical listening room can provide a ‘boost’ in the other direction that can approximately compensate for the sealed speaker’s roll-off if the speaker’s -3dB frequency is appropriate.
• Below resonance the response of the cone of a sealed speaker is still under control due to the restraining force of the air in the box, while the cone of a ported speaker is not restrained i.e. it is no longer under precise control.
• The bass reflex speaker’s output is augmented by the output of the port, which resonates in response to the movement of the back of the cone. This output is delayed, inverted and ‘smeared’ in comparison to the input signal.
• Around resonance, the sealed speaker’s cone has to move further than the ported speaker’s which implies higher distortion, but the output of the port may be heavily distorted itself, which would offset this advantage.
• A ported resonator’s output level is not completely linear, and it loses efficiency at high output levels. Overall bass output falls, and the cone’s displacement increases, resulting in higher distortion there. So the implication is that the speaker ‘pulls its punches’ on transients and sounds more brittle at higher output levels.
• There are fewer variables involved in the design of successful sealed enclosures compared to ported.

To my mind, and ears, there are few apparent disadvantages to sealed enclosures and plenty of advantages.

I’m no expert, but conventional speaker design often appears to be an exercise in ‘painting with frequency response’ and designers can be quite cavalier over what happens to the time response. If a particular technique like bass reflex or transmission line, can result in a lower -3dB point and a smaller box for the same amplifier power then they’ll use it without question. Testing with sine waves won’t show up the strange things that the port does to transients, and maybe there’s an element of not even wanting to question the orthodoxy.

As described in the article on building my own speakers, I decided to experiment with some pretty huge, sealed, active woofers. The signal is pre-corrected with DSP to linearise the phase, but there is no attempt to EQ the frequency response in-room at the bottom end.

The sonic results lead me to question many of my assumptions regarding the quality of speakers I have heard over the years. Is good bass, in fact, much more than a ‘nice-to-have’? Is it a crucial element without which an otherwise ‘perfect’ speaker simply cannot sound good? Without good bass, does the system’s frequency response appear to assume a critical importance that is, in fact, a ‘red herring’? And what of other variables? In the absence of good bass, can anyone make a meaningful judgement of whether valves are better than solid state, or vinyl better than digital, for example?

Bass is not merely a layer of paint underneath the music that gives it warmth or balance, nor is it an effect designed to shake the room. It is the foundation of the music’s dynamics and provides important information about the dimensions of the acoustic space the recording was made in. What makes us think we can discard most of it without incurring any penalty?

At the very least, good bass (and that means in the time domain, too) provides us with a realistic physical ‘hit’ from a symphony orchestra. And much of the excitement of a live rock performance lies in the sheer excessiveness of the volume and depth of the bass. By losing the bass we lose much of the excitement, plus we fail to hear some of what the performers are responding to in their playing.

There are sounds in some classical performances that are on the verge of being infrasonic. This sort of bass can be clean, with little in the way of accompanying harmonics, so that with typical speakers you simply wouldn’t be aware it was there at all. This has been a revelation to me.

These all seem like positive things, but in reality they are just a lack of negative things! Simply recreating the whole sound, rather than lopping a chunk off it or smearing it into a muddy rumble that obscures detail, makes it easier for our ears and brains to piece together the musical event that was captured in the recording.

My speakers have transformed my listening to recorded music. I no longer feel that part of the sound is wrong, or missing. I am no longer clutching at straws, worrying about non-existent cabinet resonances or whether I am using the wrong type of carpet spikes. I think that a fundamental part of this musical contentment is the completeness of the bass.

Meridian DSP7200

It would be remiss not to mention the Meridian line of DSP-active speakers. I have to admit that I love the very idea of Meridian as a company (or do I mean as a concept?). I’ve only heard one pair in my life, and they didn’t impress me, but this was at some audio show and they were being played at top volume with nondescript music – I hope to be able to hear some properly at a dealers in the near future. On paper, you could get a formidable bargain buying some secondhand on eBay, even though they might be quite a few years old, and you might, therefore, expect that their technology is out of date. But of course the technology to do this has been viable for a long time, and the basic maths hasn’t changed unless you believe that you can hear the improvement of ‘apodising filters’ (I wouldn’t like to bet on it) or higher ‘res’ (ditto).

Martin Colloms reviewed the DSP7200, and his impressions tally with other people’s reviews of DSP-active speakers:

It’s unquestionably ‘active’, with the grip, near effortless dynamic range, convincing integrity and authority that is typical of the breed. The stereo image was simply excellent, in depth width and focus. There was no aural confusion here, as it sounded almost effortlessly clear, with crisp stable imaging of believable width, coupled with stable off-axis placement where obviously phase displaced content so dictates.

The bass is unusually good, as powerful at very low frequencies as the two hard working 8.5in (216mm) bass units could supply… The bass clearly sounded ‘different’, even compared with very large and extended low frequency alternatives. Something about the 7200 got closer to the truth, with tighter control, better tune playing, and an ability to differentiate confidently between percussive and sustained bass sounds. Tracks combining both at once can tend to blur into one sound, but not so with the 7200.

Overall it sounds essentially neutral, if marginally rich and comfortable, giving a slightly distant effect that caresses rather than assaults the ears, even when playing very loud.

In the review Colloms has a moment of doubt, but resolves it by changing from the Meridian CD player to a different brand using standard S/PDIF (- I’m saying nothing…). He then says:

The sound was now so special that any thoughts about colorations, response errors et al paled into insignificance. It was fascinating to hear a speaker which so well commanded one’s emotional responses that any debate over objective criticisms now seemed nonsensical and irrelevant.

…which is somewhat similar to something I said in the post about my homebrew speakers i.e. that this DSP-active malarkey is so special, that it shows up that the things passive speaker users are imagining to be wrong with their speakers are, perhaps, red herrings; their problems go much deeper!

Piraeus DSP-based speakers

Another ‘rational’ system I didn’t know about, reported on here. I have to say that the looks don’t appeal to me all that much, but it seems that other people disagree.

Are the mid and tweeter tilted backwards..? On passive speakers this is often done for time alignment of the drivers, but of course with DSP it can be done in software.

The enclosures house the DSP crossovers (phase linear and all that) and the amplifiers.

 

 [Pictures from the manufacturers’ web site.]